Centromere

"Point centromeres" bind to specific proteins that recognize particular DNA sequences with high efficiency.

Most organisms, ranging from the fission yeast Schizosaccharomyces pombe to humans, have regional centromeres.

[2][3] Metacentric means that the centromere is positioned midway between the chromosome ends, resulting in the arms being approximately equal in length.

Submetacentric means that the centromere is positioned below the middle, with one chromosome arm shorter than the other, often resulting in an L shape.

In addition to some protein coding genes, human acrocentric p-arms also contain Nucleolus organizer regions (NORs), from which ribosomal RNA is transcribed.

However, a proportion of acrocentric p-arms in cell lines and tissues from normal human donors do not contain detectable NORs.

[5] The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation[6] and paracentric inversion.

[9] For example, human chromosome 2, which is believed to be the result of a Robertsonian translocation at some point in the evolution between the great apes and Homo, has a second, vestigial, centromere near the middle of its long arm.

Monocentric centromeres are the most common structure on highly repetitive DNA in plants and animals.

[12] The nematode, Caenorhabditis elegans, is a well-known example of an organism with holocentric chromosomes,[13] but this type of centromere can be found in various species, plants, and animals, across eukaryotes.

Holocentromeres are actually composed of multiple distributed centromere units that form a line-like structure along the chromosomes during mitosis.

[14] Alternative or nonconventional strategies are deployed at meiosis to achieve the homologous chromosome pairing and segregation needed to produce viable gametes or gametophytes for sexual reproduction.

Holocentricity has evolved at least 13 times independently in various green algae, protozoans, invertebrates, and different plant families.

The term overlaps partially with "holocentric", but "polycentric" is clearly preferred when discussing defectively formed monocentric chromosomes.

There is some actual ambiguity as well, as there is no clear line dividing up the transition from kinetochores covering the whole chromosome to distinct clusters.

Beyond "polycentricity" being used more about defects, there is no clear preference in other topics such as evolutionary origin or kinetochore distribution and detailed structure (e.g. as seen in tagging or genome assembly analysis).

In humans, the primary centromeric repeat unit is called α-satellite (or alphoid), although a number of other sequence types are found in this region.

They evolve rapidly between species, and analyses in wild mice show that satellite copy number and heterogeneity relates to population origins and subspecies.

If the centromere is inherited epigenetically from one generation to the next, the problem is pushed back to the origin of the first metazoans.

In addition, a recent assembly of the human genome has detected a possible mechanism of how pericentromeric and centromeric structures evolve, through a layered expansion model for αSat sequences.

As this process is repeated over time, the layers that flank the active centromere shrink and deteriorate.

[33] In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi.

[34] In nematodes such as Caenorhabditis elegans, some plants, and the insect orders Lepidoptera and Hemiptera, chromosomes are "holocentric", indicating that there is not a primary site of microtubule attachments or a primary constriction, and a "diffuse" kinetochore assembles along the entire length of the chromosome.